In-vitro studies suggest that 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] interacts with thymic lymphocytes. Indeed, a subset of rat and human thymocytes has been found to possess receptors for 1,25 (OH)2D3. In the rat thymus these cells belong to the PNA-negative subset and they are distinct from the glucocorticoids (GC)-sensitive cells. In addition, 1,25(OH)2D3 influences the phenotype of rat thymocytes in-vitro: 1,25(OH)2D3, added to cultures of thymocytes, increases the percentage of cells possessing the T helper phenotype. Further, 1,25(OH)2D3 decreases in a dose-dependent fashion the rate of thymocyte death. Finally, we have found that GC induce an increase of 1,25(OH)2D3 binding to cultured rat thymocytes. This proposal aims to elucidate the significance of these observations to the in-vivo situation, by testing the possibility that the Vitamin D endocrine system might play a physiologic and/or pharmacologic role in the development, maturation and function of thymic lymphocytes. Specifically, we propose to examine in Vitamin D-replete and D-deficient rats, before and after exogenous administration of 1,25(OH)2D3 and before and after antigenic challenge: 1) the anatomical and histological features of the thymus gland; 2) the cellular profile of the thymus gland, including the distribution of phenotypically distinct subpopulations of thymocytes; 3) the concentration and characteristics of 1,25(OH)2D3 receptors in thymocytes; 4) the thymocyte responsiveness to 1,25(OH)2D3 following in-vitro mitogenic stimulation; and 5) the distribution of phenotypically distinct subpopulations of T lymphocytes in the peripheral blood. Because of evidence suggesting that the thymus gland becomes atrophic with age, and evidence that the concentration of 1,25(OH)2D3 receptors in thymocytes also declines with age, we propose to compare the changes of thymic and peripheral lymphocytes, induced by antigenic challenge before and after administration of 1,25(OH)2D3, also between young and old normal rats. Finally, because of the possibility that GC and 1,25(OH)2D3 interact with each other in the regulation of immune functions, we propose to study the in-vivo effect of the combined administration of these two steroids on the above parameters (#1-5) in normal rats. The proposed studies should provide a comprehensive insight on the involvement of 1,25(OH)2D3 in the maturation and function of the thymus that could be of clinical and/or pharmacological importance.